Pertussis outbreak in a basic school in Estonia: description, contributing factors and vaccine effectiveness.

The aim of the current study was to assess the epidemiological situation concerning the emergence of a pertussis outbreak, as well as potential contributing factors and vaccine effectiveness. A retrospective epidemiological description and an analysis of the outbreak among students were performed. The basic school in Adavere had a total of 150 students in 2003. Of these, 54 cases of pertussis, with median age 12 y, all corresponding to clinical case definition, were identified with an attack rate of 36%. Regarding confirmation of the diagnosis, out of all clinical cases, 18 were confirmed by laboratory testing (2 by isolation of B. pertussis and 16 serologically based on single sera) and 36 with epidemiological linkage only. Of all the students with pertussis, 35 (65%) had received 4 doses and 6 (11%) 3 doses of DTwP vaccine; 13 (24%) students had received fewer than 3 doses or were unvaccinated. The contributing factors in generating this outbreak were close epidemiological contacts, late identification of pertussis diagnosis in the primary, secondary and later cases, as well as a too late initiated active surveillance. In this outbreak, low vaccine effectiveness and low vaccination coverage also played an important role.